1. Field of the Invention
This invention relates generally to a hydraulic steering system for a vehicle. More specifically, this invention relates to a hydraulic steering system having amplification of manually generated hydraulic pressure.
2. Description of the Prior Art
Hydraulic power steering systems generally consist of two types--hydromechanical systems and pure hydraulic power steering. This invention applies to the latter type.
In the past, pure hydraulic power steering systems provided the operator with little feel for the degree of turning taking place. Additional turning feedback was given to the operator by improving the input response to these systems. Input response was improved by using a hand operated steering pump to provide a low volume or signal flow of hydraulic fluid and amplifying the signal flow to power a hydraulic steering motor. Systems of this type are explained in SAE Papers 660238 and 720802. Amplification in these systems is provided by a control valve responsive to a pressure differential supplied by the hand operated steering pump.
U.S. Pat. No. 3,473,324 issued to Mercier shows a steering system in which the flow of fluid from the hand operated steering pump is combined with the main flow of hydraulic fluid to the hydraulically energized steering motor. It has also been taught by Baines in U.S. Pat. No. 3,463,260 and Harding in U.S. Pat. No. 3,589,465 to combine the hand generated flow of fluid with the main flow of hydraulic fluid within a control valve for flow amplification. Combining the signal flow of hydraulic fluid with the main hydraulic fluid flow increases the total amount of fluid flow into the hydraulic motor with a resulting increase in its responsiveness. In addition, these prior art references teach routing the fluid flow from the hand operated steering pump to the hydraulic motor as a means of providing emergency steering in the event of a main pump failure.
In order to further increase the responsiveness of the steering system and give the operator additional feedback on steering maneuvers, it is desirable to have the amount of amplification proportional to the input from the hand operating steering pump. U.S. Pat. No. 3,938,331 granted to Polacek et al teaches a pure hydraulic power steering system having proportionality between the flow rate of the main hydraulic pump and a hand operated steering pump. This proportionality is provided by flow meters and control valves external to the control valve for directing fluid flow to the hydraulic motor.
It is an object of this invention to provide a power steering system having a single control valve for proportionately amplifying fluid flow from a hand operated steering pump.
A further object of this invention is to combine hand generated fluid pressure and amplified fluid pressure in a single control valve for proportionately amplifying fluid flow.
It is a further object of this invention to provide a steering system that will facilitate manual steering operation in an emergency situation.